There are many electronic applications for which data needs to be transmitted at high speeds over band-limited channels. For example, data storage systems, servers, data communication systems and digital video systems all need to provide high-speed serial links over band-limited channels. This can be accomplished by providing a transmitter at one end and a receiver at the other end of a “telecommunications channel” or “communication link.”
In telecommunications, inter-symbol interference (ISI) is a form of signal distortion wherein a transmitted symbol interferes with subsequent transmitted symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication channel less reliable. That is, the presence of ISI may introduce errors at the receiver output. Therefore, in the design of telecommunication systems, an objective is to minimize the effects of ISI and thereby deliver the digital data to its destination with the lowest error rate possible, e.g. with the best signal-to-noise ratio (SNR). Ways to light inter-symbol interference include, for example, adaptive equalization techniques.
For example, Continuous Time Linear Equalizers (CTLE) can be used in communication links to compensate for the channel's frequency dependent loss which causes ISI. The CTLE's equalization parameters are adjusted to minimize the ISI and jitter of the CTLE's output. This adjustment is typically set manually for each channel and susceptible to environmental, part-to-part and channel manufacturing, variability.
Methods exist to automatically adjust the CTLE equalization parameters for implementations which include a clock and data recovery circuitry. For example, the circuits described in “Electrical signal processing techniques in long-haul fiber optic systems,” Winters, J. H. & Gitlin, R. D., AT&T Bell Lab., IEEE Transactions on Communications, September 1990. These circuits recover a clock that is phase-locked to the input signal. Utilizing the recovered clock, the quality of the output “eye” can be measured at its center to optimize the CTLE, equalization parameters.
Analog methods exist that balance the high and low frequency content of the output waveform. These analog methods do not directly measure the output eye quality. For example, the MAX3805 Adaptive Receive Equalizer marketed by Maxim Integrated Products of Sunnyvale, Calif. is a continuous time linear equalizer (“CTLE”) which utilizes a frequency domain analog technique to reduce the effects of ISI. By way of further example, the article “1-mW 12-Gb/s Continuous-Time Adaptive Passive Equalizer in 90-nm CMOS”, by Doug Hun Shin, Ji Fun king, Frank O'Mahony and C. Patrick Yue, CICC, 2009, describes a continuous time linear equalizer (“CTLE”) which also reduces the effects of ISI.
Both of the references cited above measure energy in two frequency bands and control the equalization of a CTLE to match the expected energy distribution for a random non-return-to-zero (“NRZ”) data pattern at an expected data rate. ISI is reduced with a subsequent increase in SNR by properly controlling the equalization of the CTLEs.
While the apparatus described in these references operate well for their intended applications they also exhibit certain drawbacks under some circumstances. For example, these prior apparatus are susceptible to the analog filter's accuracy, offsets and production variations. Furthermore, these prior apparatus require an a priori knowledge of the serial data rate and expected distribution of energy of the transmitted symbols. Also, prior apparatus which utilize a phase-lock-loop (PPL) to recover the clock from the transmission consume a substantial additional amount of power.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.